The present invention relates to an aromatic polyamide film suitable for use as a base film for a high-density magnetic recording medium. More specifically, it relates to an aromatic polyamide film which is excellent in handling properties such as strength, slipperiness and winding properties, and in chipping resistance, and when used for a high-density magnetic recording medium, the magnetic recording medium has excellent electromagnetic conversion characteristics, drop out resistance, output reduction resistance and traveling durability.
In recent years, remarkable progress in a higherdensity has been made in magnetic recording medium, as exemplified by the development and implementation of a metal thin film magnetic recording medium produced by forming a ferromagnetic metal thin film on a non-magnetic base film by a plating method or a physical deposition method such as vacuum vapor deposition or sputtering, and a thin layer coated magnetic recording medium produced by coating needle-shaped magnetic powders such as metal powders or iron oxide powders to a thickness of 2 xcexcm or less.
Known examples of the former include a Co deposited tape (JP-A 54-147010) and a vertical magnetic recording medium comprising a Coxe2x80x94Cr alloy (JP-A 52-134706) whereas known examples of the latter include an extremely thin layer coated magnetic recording medium for high-density magnetic recording (Technical Report MR 94-78 issued by The Institute of Electronics, Information and Communication Engineers (1995-02)) and the like. Since a coated magnetic recording medium of the prior art (a magnetic recording medium produced by coating a mixture of magnetic powders and an organic polymer binder on a non-magnetic base film) has low recording density and a long recording wavelength, the thickness of its magnetic layer is as thick as at about 2 xcexcm or more. On the other hand, a ferromagnetic metal thin film formed by thin film forming means such as vacuum vapor deposition, sputtering or ion plating has an extremely small thickness of 0.2 xcexcm or less. An extremely thin layer coated medium as thin as 0.13 xcexcm has been proposed ever though it comprises a non-magnetic base layer.
In line with this, a non-magnetic base film is also desired to be thin and required to have sufficient strength in the reduced thickness and rigidity which provides an appropriate state of contact with a magnetic recording head. In this respect, an aromatic polyamide film has excellent strength and rigidity in a small thickness causing to high Young""s moduli and hence, has been recently used as a base film for a high-density magnetic recording medium frequently.
Meanwhile, in the above high-density magnetic recording medium, the surface condition of the base film exerts a great influence on the surface properties of a magnetic layer. Particularly, in the case of a metal thin film magnetic recording medium, the surface condition of the non-magnetic base film appears directly as the uneven surface of the magnetic layer (magnetic recording layer), thereby causing noise in recording and reproduction signals. Therefore, it is desirable that the surface of the non-magnetic base film be as flat as possible.
On the other hand, from the viewpoints of the formation of a non-magnetic base film and handling properties such as transfer, winding-up and winding-off in the step of film formation, when the surface of the film is too flat, slipperiness between films worsens, a blocking phenomenon occurs, and the roll formation of the base film deteriorates, with the consequence that the yield of products lowers and further, it invites boosting of the production cost of a product. Therefore, if considered from the viewpoint of the production cost, it is desirable that the surface of the non-magnetic base film be as rough as possible.
Further, in the case of a metal thin film magnetic recording medium, a serious problem encountered when it is actually used is the unsatisfactory running properties of the surface of a metal thin film. In the case of a coated magnetic recording medium produced by mixing magnetic powders into an organic polymer binder and coating the resulting mixture on a base film, the running properties of the magnetic layer surface can be improved by dispersing a lubricant in the binder. However, in the case of a metal thin film magnetic recording medium, such measure cannot be taken, and it is extremely difficult to maintain stable running properties. The metal thin film magnetic recording medium has a defect that it is inferior in running properties especially under high-temperature and high-humidity conditions.
Under the circumstances, to produce a high-quality high-density magnetic recording medium, the above two contradictory requirements must be satisfied at the same time.
As means for realizing this, there have been proposed (1) a method for forming a discontinuous coating film by applying a specific coating to the surface of a film (see JP-B 3-80410, JP-A 60-180839, JP-A 60-180838, JP-A60 -180837, JP-A 56-16937 and JP-A 58-68223), (2) a method for forming a continuous coating film having a fine uneven surface on the surface of a film (see JP-A 5-194772 and JP-A 5-210833), (3) a method for forming different front and rear surfaces by such a technology as coextrusion (see JP-A 2-214657 and JP-B 7-80282), and (4) a method which combines the above methods (1) and (3) or (2) and (3) (see JP-A 3-73409).
However, although the above conventional methods (1), (2) and (4) for forming a discontinuous coating film and a continuous coating film having a fine uneven surface can solve such problems as blocking and insufficient slipperiness between films, it is difficult to uniformly disperse fine inert particles into a coating film and coarse protrusions are liable to be formed due to agglomerated particles. They still have, therefore, a problem in that the quality of the resulting magnetic tape is instable, as exemplified by deterioration of the electromagnetic conversion characteristics caused by the coarse protrusions. The agglomerated particles are chipped off by their contact with guide rolls in the step of film formation more easily than monodisperse particles, are adhered to and accumulated on a base film to form protrusions, and cause a drop out when it is formed into a magnetic tape.
Though inorganic particles have high hardness and are rarely deformed and accordingly, they are excellent in cleaning a magnetic head and it is easy to obtain fine particles having different sizes, the particles have poor affinity with a polymer and are liable to fall off from the film. On the other hand, organic particles have excellent affinity with a polymer, but they have lower hardness than inorganic particles and the whole particles undergo deformation by heat or mechanical friction. Therefore, the electromagnetic conversion characteristics of a tape containing the organic particles become deteriorated when the tape runs repeatedly.
Further, a thermal load on a tape is becoming greater and greater with high-density packaging as a reduction in the size of a hardware increases. Since the pitch of magnetic recording tracks of a high-density magnetic recording medium in particular is as very narrow as around 10 xcexcm, the dimensional change of a base film caused by thermal history leads to track displacement, resulting in deterioration in electromagnetic conversion characteristics.
It is an object of the present invention to provide an aromatic polyamide film which is excellent in chipping resistance and winding properties in the step of film formation and when it is used in a deposited metal thin film magnetic recording medium, the resulting magnetic recording medium has excellent electromagnetic conversion characteristics, drop out resistance, output reduction resistance and traveling durability.
It is another object of the present invention to provide a laminate film for a high-density magnetic recording medium which prevents the formation of agglomerated particles causing a drop out, and can give a base film for a deposited metal thin film magnetic recording medium that has excellent electromagnetic conversion characteristics and causes few drop outs, and enables the production of a large-capacity magnetic recording medium.
It is still another object of the present invention to provide a transferable thin film which has useful surface flatness and excellent transferability as a base film for a high-density magnetic recording medium and can satisfy contradictory requirements of prevention of projection (being pushed up) of protrusions and winding easiness in a calendering step at the same time.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by an aromatic polyamide film for a high-density magnetic recording medium (may be referred to as xe2x80x9cfirst aromatic polyamide film of the present inventionxe2x80x9d hereinafter) which film comprises:
(A) a biaxially oriented aromatic polyamide base film of an aromatic polyamide containing substantially no inert fine particles and having a total of Young""s moduli in two directions crossing at right angles each other on the film plane of at least 2,000 kg/mm2; and
(B) a coating layer comprising inert fine particles having an average particle diameter of 5 to 100 nm and a binder resin and formed on at least one side of the base film, the average particle diameter (dB: nm) of the inert fine particles and the thickness (tB: nm) of the coating layer satisfying the following expression (1):
0.05xe2x89xa6tB/dBxe2x89xa60.8xe2x80x83xe2x80x83(1)
and the coating layer having protrusions on its surface at a density of 1xc3x97106 to 1xc3x97108/mm2.
Secondly, the above objects and advantages of the present invention are attained by an aromatic polyamide film for a high-density magnetic recording medium (may be referred to as xe2x80x9csecond aromatic polyamide film of the present inventionxe2x80x9d hereinafter) which film comprises:
(A) a biaxially oriented aromatic polyamide base film of an aromatic polyamide containing substantially no inert fine particles and having a total of Young""s moduli in two directions crossing at right angles each other crossing on the film plane of at least 2,000 kg/mm2; and
(B) a coating layer made comprising inert fine particles, a surfactant and a binder resin and formed on at least one side of the base film, the surfactant consisting of a first surfactant having an HLB value of 10 to 14 and a second surfactant having an HLB value of 16 to 18.5, the average HLB value of the first surfactant and the second surfactant being 15 to 18, and the contents of each of the first surfactant and the second surfactant in the coating layer being 0.1 to 15 wt % and 10 to 40 wt % in terms of solids.
Thirdly, the above objects and advantages of the resent invention are attained by a transferable aromatic polyamide film for a high-density magnetic recording medium (an aromatic polyamide film comprising a laminate base film (A1) may be referred to as xe2x80x9cthird aromatic polyamide film of the present inventionxe2x80x9d and an aromatic polyamide film comprising a biaxially oriented aromatic polyamide base film (A2) may be referred to as xe2x80x9cfourth aromatic polyamide filmxe2x80x9d hereinafter), which film comprises:
(A) (A1) a laminate base film which comprises a biaxially oriented aromatic polyamide film of an aromatic polyamide containing substantially no inert fine particles, a total of Young""s moduli in two directions crossing at right angles each other on the film plane being at least 2,000 kg/mm2, and a first coating layer of 0.01 to 45 wt % of inert fine particles having an average particle diameter of 1 to 300 nm and a binder resin and formed on one side of the polyamide film, or
(A2) a biaxially oriented aromatic polyamide base film of an aromatic polyamide containing internal inert fine particles composed of a compound represented by the following formula (2):
M1(OX)x
wherein M1 is the elemental metal of the group Ia or IIa of the periodic table and x is 1 or 2, or a compound represented by the following formula (3):
M1(CO3)y
wherein M1 is the same as defined above and y is xc2xd or 1, and having an average particle diameter of 5 to 1,500 nm, a total of Young""s moduli in two directions crossing at right angles each other on the film plane being at least 2,000 kg/mm2; and
(B) a second coating layer comprising two types of inert fine particles having different average particle diameters and a binder resin and formed on a side devoid of the coating layer of the laminate base film (A1) or one side of the biaxially oriented aromatic polyamide base film (A2), the two types of inert fine particles consisting of first inert fine particles having an average particle diameter of 100 to 1,000 nm and second inert fine particles having an average particle diameter of 5 to 100 nm, the ratio of the average particle diameter of the first inert fine particles to the average particle diameter of the second inert fine particles being 1.2 or more, the contents of each of the first inert fine particles and the second inert fine particles in the second coating layer being 0.01 to 40 wt % and 1 to 70 wt % in terms of solids, the total of the contents of both particles being 75 wt % or less, and the thickness of the second coating layer being 1 nm or more but not more than the average particle diameter of the second inert fine particles.